The demand for electric and hybrid electric vehicles continues to grow as governmental agencies, private organizations and individual consumers strive to conserve resources, reduce energy costs and lower carbon emissions. Electrified vehicles have an electric drive system that includes an electrical machine to assist or replace the internal combustion engine of a conventional vehicle, a power conversion system configured to provide energy to the electric machine, and a machine controller configured to control machine operation. Due to its high power/torque density, its high efficiency, and its high reliability, a permanent magnet synchronous motor (PMSM) is a popular choice for electric vehicle drive systems. Most machine controllers used with PMSMs in electric vehicle applications implement field-oriented control (FOC) techniques due to the high dynamic response and performance they can provide. However, FOC systems can be rather complex, requiring continuous rotor position information, current regulators, transformations between rotating and stationary coordinate systems, and pulse width modulation (PWM) generators. In addition, FOC systems can be sensitive to parameter fluctuations, external disturbances and load changes.
Recently, increased consideration is being given to the use of interior PMSM (IPMSM) machines in electric vehicle applications. IPMSMs are already widely used in high-performance drives ranging from servos to traction applications. The development of highly coercive permanent magnet materials like neodymium iron boron (NdFeB) at lower costs makes IPMSMs, with their increased energy density, an attractive option for electric propulsion systems. In addition, IPMSMs provide good dynamic performance with a high torque/inertia ratio, are highly efficient, and are also highly reliable. Unlike the inductance characteristics of a typical PMSM, the IPMSM q-axis inductance can be much larger than the d-axis inductance. This disparity increases the efficacy of flux weakening operations at IPMSMs, and enables them to provide an extended constant power range capability over that of PMSMs. Such capability is advantageous in electric vehicle applications, as studies have shown that it can allow the use of power inverters with lower volt-ampere ratings, and abrogate the need for multiple gear ratios.
DTC, originally introduced for induction motor control, offers direct, independent control of the stator flux linkage and the electromagnetic torque of an electric machine. Because the electromagnetic torque of an IPMSM is proportional to the angle between the stator and rotor flux linkages, DTC, with its good dynamic response, is an attractive option for IPMSM propulsion systems in electric vehicles. In general, DTC is designed to control torque and flux linkage by applying a voltage vector to an inverter configured to provide alternating current to the electric machine. The particular voltage vector applied is dependent on the outputs of a torque hysteresis controller and a flux hysteresis controller. A DTC system can include a voltage table that associates various combinations of hysteresis controller outputs with a voltage vector that is expected to result in the fastest electromagnetic torque response. Two significant control parameters for this table-based DTC solution are the torque and flux hysteresis controller bands which need to be calibrated or tuned to provide a desired level of performance. While small bands may be desired for higher machine output accuracy, they are generally not achievable given the physical limitations of drive system components. On the other hand, large bands, more easily achievable, are prone to noise, vibration and harshness problems which can adversely impact performance and disappoint and/or frustrate an operator. Unfortunately, there is no universal or standard method for specifying hysteresis controller bands for a particular application. Instead, determination of controller bands is essentially a trial and error process which can be costly and inefficient.